Research
Sedimentary rocks record how Earth balances the chemical demands of life and its environment. My research uses them to unravel Earth's evolving biogeochemistry.
Magnesium carbonates on Earth and Mars
I am currently leading a team studying serpentinite weathering and magnesium carbonate precipitation in soils and alluvium to understand facets of the ancient Martian hydrosphere and carbon cycle. We are studying the geochemistry and architecture of ultramafic- and sediment-hosted magnesite mineralization in Australia at scales detectable by the Perseverance rover, which is exploring analogous deposits in Jezero Crater, Mars. On Earth, this work provides opportunities to map source-to-sink controls on carbon and solute fluxes from terrestrial environments to coastal mangroves, and to the oceans via the Great Barrier Reef.
This work is funded by:
- Simons Foundation Life Sciences Project Award 668346
- Caltech Center for Comparative Planetary Evolution
Preservation of microbial mats on peritidal ooid sand
Sedimentary rocks called microbialites preserve physical and chemical clues about the interaction between microbial life and its environment throughout the geologic record. However, how mats lithify—to form microbialities—is incompletely understood, particularly in marine environments that form the bulk of the rock record. On Little Ambergris Cay, Turks & Caicos Islands, we document an extensive modern microbial mat ecosystem that destroys nearly all traces of itself in the shallowest sedimentary record, despite widespread carbonate mineral precipitation from the surrounding environment. Using a variety of observational approaches and measurements, we conclude that two common phenomena in tropical carbonate platform environments—highly productive benthic ecosystems and advective tidal pumping—combine to prevent preservation of the microbial mats. These phenomena are likely common characteristics of many ancient microbialite facies associations. Our results suggest that there is a significant taphonomic bias against fossilizing signs of the most productive microbial ecosystems, explain why modern microbialite-forming environments are marginal and possibly unrepresentative of typical ecosystems, and guide the search for ancient strata that indeed might preserve evidence of microbial life (Present et al., Nature Communications, 2021; web/PDF).
This work is funded by:
- Agouron Institute
- Simons Collaboration on the Origins of Life
Anaerobic metabolisms affect the carbon cycle and the rock record
I am using trace sulfate in calcite and dolomite (carbonate-associated sulfate, CAS) to explore how local processes affect proxies of global biogeochemical cycles and how those processes themselves leave signatures of biogeochemical changes. CAS is a proxy for seawater sulfate, and informs the global redox balance of oxygen and carbon. However, I demonstrated that even in well-preserved limestone successions spanning the end-Ordovician mass extinction interval on Anticosti Island, Quebec, postdepositional diagenesis can severely overprint primary seawater signals (Present et al., EPSL, 2015; web/PDF). To understand how and where this diagenesis may obscure our records of past ocean composition, I used well-exposed and correlatable Late Permian strata in the Guadalupe Mountains, Texas to examine the importance of local environmental processes on the CAS proxy (Present et al., Sedimentology, 2019; web/PDF).
Together, this body of work refines our understanding of diagenetic variability in the carbonate rock record. My work demonstrates that, in addition to preserving the history of ancient seawater's composition, the rock record can be used to reconstruct microbial sulfate reduction in ancient pore fluids, which today is responsible for as much as a third of all marine organic carbon remineralization. These findings inform long-term changes in the sulfur isotopic composition of sulfate in ancient oceans, because CAS is the most widespread and continuously-deposited proxy archive but is shaped by both primary and postdepositional processes (Present et al., GRL, 2020; web/PDF). As an ongoing project, I am working to understand Paleozoic sulfur cycle changes by extending the Ordovician-Silurian CAS sulfur isotope record using exquisitely well-preserved brachiopod calcite. Please contact me if you would like to share brachiopods!
This work is funded by:
- American Chemical Society New Directions Grant #53994-ND2
- Society for Sedimentary Geology (SEPM) Student Research Grant
- Society for Sedimentary Geology (SEPM) Student Travel Grant
Metal sulfides in the environments of early life
I also study metal cycling during diagenesis, with diverse geobiological applications. For example, in a Mesoproterozoic base metal sulfide deposit (Belt Supergroup, MT) copper, barium, and sulfur cycling at an ancient hydrothermal vent created unique, beautiful vent structures formed as microbial life interacted with its anoxic deep-ocean environment (Present et al., GSA Bull., 2018; web/PDF). In an ongoing collaboration, I am now working on modelling metal sulfide aqueous speciations and their competitive ligation with metalloenzymes used by Earth's earliest microorganisms.
This work is funded by:
- Agouron Institute
- NASA Astrobiology Institute
